\begin{tikzpicture}
    \begin{scope}
        \pgfmathsetmacro{\r}{1.3}
        \coordinate (A) at (0, 0);
        \coordinate (C) at (-1, 1.5);
        \coordinate (B) at (2, 0);
        \draw [thick] (B)
            -- (A) node [below] {$A$}
            -- (C) node [above] {$C$} node [pos=0.6, below] {$b$}
            -- ($(C) + (335:\r)$) node [midway, above] {$a$};
        \draw pic [draw, angle radius=0.8em] {angle=B--A--C};
        \draw [densely dashed] (C) + (290:\r) arc (290:350:\r);

        \node at (1, -1) {$a \leqslant b$};
        \node at (1, -1.5) {无解};
    \end{scope}

    \begin{scope}[xshift=5cm]
        \pgfmathsetmacro{\a}{3}
        \coordinate (A) at (0, 0);
        \coordinate (C) at (-1, 1.5);
        \coordinate (P) at (2.5, 0);

        \path [name path=ap] (A) -- (P);
        \path [name path=cb] (C) + (270:\a) arc (270:360:\a);
        \path [name intersections={of=ap and cb, by={B}}];

        \draw [thick] (B)
            -- (A) node [below] {$A$}
            -- (C) node [above] {$C$} node [midway, below] {$b$}
            -- (B) node [below] {$B$} node [midway, above] {$a$};
        \draw (B) -- (P);
        \draw pic [draw, angle radius=0.8em] {angle=B--A--C};
        \draw [densely dashed] (C) + (310:\a) arc (310:340:\a);

        \node at (1, -1) {$a > b$};
        \node at (1, -1.5) {一解};
    \end{scope}
\end{tikzpicture}

